1. Field of the Invention
The present invention relates to a mobile communication device for carrying out communications while moving among a plurality of interconnected networks such as mobile telephone network, Internet, locally connected device network, and a router device provided in a mobile communication network for providing the Internet service.
2. Description of the Related Art
In conjunction with the rapid spread of portable telephones and the Internet, the Internet service using a portable telephone as a terminal has been developed by the mobile carriers. This is a service in which a module for processing a communication protocol called TCP/IP used in the Internet is provided in the portable telephone so as to enable WWW servile, e-mail delivery, still/video image transfer, etc. This service has an advantage in that it becomes possible to freely access not just information closed within the communication service provider but also various information available on the Internet that is spreading worldwide, and for this reason the number of users of this service is increasing rapidly.
Now, the portable telephone Internet service that has an enormous number of users encounters a serious problem regarding how to assign addresses to terminals. Currently, the Internet faces with the problem of IP address shortage in view of the ever increasing number of nodes (routers and terminals), and in the current IPv4, it has been customary to use an address system called private address that is closed within an organization for accesses in an enterprise network or the like and to use global addresses only for accesses to the external of the organization by utilizing the address conversion. Similarly, in the case of the portable telephone Internet service, it is currently customary to use the private address within the cellular phone network by providing an address conversion device (NAT: Network Address Translator) at a border between the IP packet network of the portable telephone company and the global Internet.
On the other hand, the next generation IP specification called IPv6 is currently under the discussion. The IPv6 can accommodate far more nodes compared with the IPv4 by expanding the IP address from a 32 bits width as used in the IPv4 to a 128 bits width. Similarly, in the case of the portable telephone Internet service, it is possible to assign unique global addresses to all terminals by adopting the IPv6. In this way, communications using a seamless end-to-end connection that does not require a relay device such as NAT on the communication path can be realized by using a wide address space, and it is expected to be advantageous in many aspects including the security and the communication quality guarantee.
The IPv6 has various advantages as described above, but from a viewpoint of operating the actual portable telephone Internet, for example, always assigning a global address to each terminal can cause a problem. Namely, when a global address is assigned to a terminal, this terminal becomes accessible from all nodes on the Internet so that this terminal can potentially be a target of attacks such as that for sending a large amount of unnecessary packets, for example.
However, a provision of providing a packet filter or a firewall on a route in order to prevent such attacks is not preferable because it goes against the essence of the IPv6 that transparent end-to-end communications are possible. Also, a more passive provision of not registering a name of a terminal to a name server in order to prevent attacks from the external does not provide a full solution to the problem because the number of applications that require the resolution of a name of a terminal itself such as VoIP is increasing.